Japanese Patent Application Laid-Open Publication No. 2012-027198 (Patent Document 1) describes an optical semiconductor device including an optical waveguide formed of a part of a semiconductor layer, a first impurity region which is formed in the semiconductor layer on one side of the optical waveguide and to which an impurity of a first conductivity type is introduced and a second impurity region which is formed in the semiconductor layer on the other side of the optical waveguide and to which an impurity of a second conductivity type opposite to the first conductivity type is introduced.